1. Field of the Invention
The present invention generally ribbon cassettes.
2. Related Art
Impact printing is a well-established art that grew out of the typewriter industry, which was adapted to accommodate the advent and development of computer technology. As such, the basic printing technique has remained the same: an ink-impregnated ribbon is juxtaposed between printable media (paper, film, card stock, etc.) and an impact hammer. The media is positioned against a platen so that when the hammer strikes the ribbon, ink is transferred from the ribbon into the media, producing an ink mark on the media. As the speed of printing has increased, so has the need for greater ink capacity and durability in the supply ribbon. This demand has been addressed in a variety of ways, including employing a longer and/or wider ribbon web, improving the durability of the ribbon, improving the quality of the ink, and increasing the volume of ink available by re-inking the ribbon web from a reservoir of ink.
There have been two basic techniques developed for storing and delivering the ribbon web to the printing station in the printer. The first method employed was, and continues to be, to store the length of ribbon within two spools. As printing occurs, the ribbon spools reciprocally rotate, continuously transferring the ribbon back-and-forth between the two spools. As the ribbon dwells on one or the other spool, ink diffuses from the more concentrated areas on the ribbon web to those areas where ink was transferred off of the ribbon through the printing action. However, due to high ink viscosity and relatively low ink volume, transfer through diffusion is slow and limited such that for anything other than evenly distributed printing patterns, as the inked ribbon approaches mid-life, noticeable variations of print density can appear in the printed media due to ink concentration gradients in the supply ribbon.
The second method of storing and delivering inked ribbon to the printing station significantly improves on the shortcomings of the first technique. This second method can be referred to as a ribbon cassette and can be described basically as a box within which inked ribbon—in the form of a mobius loop—is stored, and delivered to the print station by continuously pulling the ribbon in one direction through a series of rollers and guides. Because of the mobius half-twist, even with uneven printing patterns, ink tends to be more evenly transferred out of the ribbon over the life of the ribbon cassette.
However, typically, designs have inherent limitations and disadvantages, and the mobius cassette is no exception. Some of the disadvantages inherent in cassette designs include: 1) ribbon tracking problems (where the ribbon drifts laterally out of alignment) due to tension variation across the width of the ribbon, 2) ribbon edge damage due either to abrasion against wearing plastic surfaces or to impacts from printing hammers in the event that ribbon tracking errors bring the edge too close to the printing zone, and 3) ribbon jamming problems where the ribbon becomes wedged into confined spaces in the cassette resulting in excessive ribbon tension such that the drive mechanism cannot free the wedged ribbon.
Typically a motor is employed to drive the ribbon motion. To the motor shaft is attached a plastic gear or pinch roller, either of which is operated against a counter-rotating gear or roller (which is radially loaded against the drive roller), such that the ribbon, which is threaded in between the two, is forcibly pulled from the printing mechanism area into a holding volume or “stuff-box” within the cassette housing. At the far end of the stuff-box area is arranged a narrow gate through which the exiting ribbon loop continues to flow as an isolated, individual web. Passing through the gate, the ribbon is then guided over two triangular walls positioned parallel to each other and separated by a horizontally oriented gate so that the ribbon web is flipped through a mobius twist as it passes over the edges of these features. Once half-twisted in this fashion, the ribbon passes through a pinch-point (usually comprised of a leaf spring flexed against a vertical rib edge in the cassette housing), which provides sufficient back-tension in the ribbon web so as to ensure accurate translation through the print station. The ribbon web continues through the print station and back into the cassette entrance point, thus completing the loop.
The nature of the ribbon configuration in the stuff-box area is one of randomly oriented stacks of contiguous ribbon segments, folded back-and-forth in “S” like patterns, and connected “head-to-tail” by somewhat straighter segments of ribbon. Both the orientation and length of these stacked patterns are random, such that as the ribbon exits the stuff-box area, a great many differences in tension, approach angle, and friction from adjacent ribbon folds occur in that exiting segment. In the absence of any obstacles between the stuff-box area and the exiting gate, these serpentine segments of ribbon would become pressed up against the exit gate and would be dragged into the gate by the exiting web, thereby causing the ribbon to jam. This phenomenon of ribbon dynamics as the ribbon web exits a stuff-box through a narrow isolation gate, makes the task of isolating the single exiting web from the rest of the stack rather problematic. It is therefore necessary to design structural obstacles, such as ribs in the ceiling and floor and converging sidewalls so as to retard the advance of the stuffed ribbon, allowing only the exiting segment to advance to the exit gate. These approaches (ribs and converging sidewalls) are well known in the art.
Intuitively, the features designed to retard the ribbon stack from entering the gate may be more or less effective in preventing jams. A given design may be adequate under certain conditions, but become less effective under different conditions. Experience shows that one condition limiting the effectiveness of these jam-prevention features is the density of the ribbon (i.e. the amount or length) packed into the stuff box area. There is a natural limit as to the amount of ribbon that can be stuffed into a given cassette, due to the amount of pressure that the ribbon “pack” applies to the leading boundary of ribbon as it exits the stuff-box area. Once this limit is reached, a jam in the exit gate area becomes highly probable because as the density of the ribbon pack increases, more and more energy is stored in the ribbon folds—much like a compression spring. At the exit gate area, as local stacks of ribbon are removed from the pack, more and more energy is stored in the remaining local stacks, so that when they in turn become released, they spring forward rapidly. This increase in velocity and density of ribbon folds in the vicinity of the ribbon retarding features can result in multiple folds getting past those features and entering the gate thus causing a jam.
In the past, there have been numerous efforts to mitigate the propensity for jamming in the exit gate, including one or more shallow walls or ribs protruding up from the floor and down from the cover. The function of the rib (or ribs) so oriented is to block the advance of the packed ribbon mass, while allowing the single exiting web to advance by forcing it to buckle so it can pass by the ribs.
Other ideas include the positioning of angled side walls, and schemes for laying-down the folding ribbon in more uniform patterns (e.g., as disclosed in U.S. Pat. Nos. 4,645,364; 4,645,388; and 4,212,420). Uniform ribbon packing schemes are inherently problematic and unreliable, and tend to add excessive cost to the manufacture of such cartridges.
FIG. 1 illustrates some of the physical characteristics of ribbon packing and translation through an exit gate of a conventional ribbon cassette. As can be seen in the illustration, packing forces transfer through the stuffed stack of ribbon, pressing the leading folds of ribbon against retaining walls or ribs 102 just “upstream” of an exit gate 104. As ribbon is extracted through gate 104, a void 106 forms in the stuff box area, leaving the remaining ribbon in contact with the retaining wall to carry all of the packing forces. The resulting increase in pressure on the leading folds of ribbon forces some of them past retaining ribs 102, which allows them to expand rapidly into the area immediately in front of exit gate 104. Occasionally, this pulse of motion in the expanding bundle of ribbon results in one or more folds advancing into the immediate vicinity of exit gate 104, potentially initiating a ribbon jam as the exiting web drags one of them into gate 104.
This condition is exacerbated by increasing the amount of ribbon in the stuff box area and is reduced by lowering the amount of ribbon stuffed into the cartridge. Since it is often desirable to maximize the amount of ribbon in the cartridge, an inexpensive means of preventing this mode of ribbon jamming while increasing the length of ribbon stored would be advantageous.